From mboxrd@z Thu Jan 1 00:00:00 1970 From: "H. Peter Anvin" Subject: Re: On the subject of RAID-6 corruption recovery Date: Fri, 04 Jan 2008 16:45:21 -0800 Message-ID: <477ED321.6000708@zytor.com> References: <4774663C.5090609@zytor.com> <59840.88.217.65.202.1199491167.squirrel@www.e18.physik.tu-muenchen.de> <477EC96D.80505@zytor.com> <51481.88.217.65.202.1199493668.squirrel@www.e18.physik.tu-muenchen.de> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <51481.88.217.65.202.1199493668.squirrel@www.e18.physik.tu-muenchen.de> Sender: linux-raid-owner@vger.kernel.org To: Thiemo Nagel Cc: Linux RAID Mailing List List-Id: linux-raid.ids Thiemo Nagel wrote: >>> For errors occurring on the level of hard disk blocks (signature: most >>> bytes of the block have D errors, all with same z), the probability for >>> multidisc corruption to go undetected is ((n-1)/256)**512. This might >>> pose a problem in the limiting case of n=255, however for practical >>> applications, this probability is negligible as it drops off >>> exponentially with decreasing n: >>> >> That assumes fully random data distribution, which is almost certainly a >> false assumption. > > Agreed. This means, that the formula only serves to specify a lower limit > to the probability. However, is there an argumentation, why a pathologic > case would be probable, i.e. why the probability would be likely to > *vastly* deviate from the theoretical limit? And if there is, would that > argumentation not apply to other raid 6 operations (like "check") also? > And would it help to use different Galois field generators at different > positions in a sector instead of using a uniform generator? > What you call "pathologic" cases when it comes to real-world data are very common. It is not at all unusual to find sectors filled with only a constant (usually zero, but not always), in which case your **512 becomes **1. It doesn't mean it's not worthwhile, but don't try to claim it is anything other than opportunistic. -hpa